1. Field of the Disclosure
The present disclosure generally relates to X-ray apparatus, and more particularly to apparatus and methods for rapidly switching the energy spectrum of diagnostic X-ray beams.
2. Description of the Background Art
Diagnostic X-ray imaging and X-ray Computed Tomography (CT) are typically performed with X-rays generated by bombarding a metal plate, or anode, with electrons that have been accelerated across a potential difference, typically in the range from about 10 kilovolts to about 140 kilovolts, or kVp. The diagnostic image is formed when a patient is positioned between the X-ray source and an imaging device. In static imaging, the image is a map of the energy deposited by the X-rays while the patient and the device do not move. In CT, the image is made by a tomographic reconstruction of measurements acquired in many orientations of the X-ray source, which revolves around the patient while the patient bed is advanced or retracted.
X-rays emerge from their source with energies ranging from nearly 0 keV up to the full energy of the electron beam. Since the radiation of lowest energy is almost entirely absorbed in the patient, thus exposing the skin to ionizing radiation without helping to build the diagnostic image, the X-rays are typically filtered by placing an absorber material between the anode and the patient. That absorber is often called a filter. Other materials in the path of the X-rays also contribute to the filtration of the beam, for example the exit window of the X-ray tube and circumambient oil in the case of a rotating tube (e.g., the Straton tube, as disclosed in commonly-owned U.S. Pat. No. 6,084,942).
Many properties of the diagnostic image are characterized by the energy content of the X-rays. This is determined mainly by the kVp setting and the type of filtration. When one can make two or more X-ray images in rapid succession, with a different energy spectrum in each case, additional information is acquired. In angiography, this arrangement allows the physician to visualize vessels filled with an X-ray contrast medium. In CT, the information provided by multiple-energy imaging allows a better discrimination between such contrast media and human bone tissue, which may be useful in the case of Positron Emission Tomography (PET)/CT, where attenuation maps are derived from the CT images.
In the case of PET/CT and also Single Photon Emission Computed Tomography (SPECT)/CT, a more accurate PET or SPECT attenuation correction is realized when the amount of contrast material in soft tissue, blood pool, and the gastrointestinal tract can be accurately determined. These applications provide the ability to distinguish bone from contrast material.